Very.
The Sum function or the AutoSum tool.
One?
If you're going to pick one, I vote for zero. But zero is rational, so it doesn't really count. This question is BS, essentially.
36 = 62 = Triangle(8) The triangle number formula: T(n) = (n2+n)/2. Here is T(8) graphically: . 1 = T(1) = 1 .. +2 = T(2) = 3 ... +3 = T(3) = 6 .... +4 = 10 ..... +5 = 15 ...... +6 = 21 ....... +7 =...